1. Field of the Invention
The present invention relates to a stencil printing apparatus, and more particularly, to a stencil sheet clamping construction for mounting a leading end portion of a stencil sheet to the outer circumferential surface of a printing drum of a rotary stencil printer.
2. Description of the Prior Art
In Japanese Patent Laid-open Publication 61-104854 based upon an application filed by the same assignee as the present application there has been proposed a stencil sheet clamping construction of a stencil printing drum, comprising a base portion provided at an outer circumferential surface of the stencil printing drum along a generatrix thereof, a press plate provided on the base portion to be selectively pivotable about a pivot axis extending along the base portion between a closed position laid over the base portion and an open position turned oppositely therefrom, means for selectively driving the press plate between the closed position and the open position, and an elastic film belt extending circumferentially relative to the stencil printing drum over an upper surface of the base portion with a first end portion thereof being fastened to the based portion at an edge portion thereof remote from the pivot axis of the press plate and a second end portion thereof opposite to the first end portion being turned around the pivot axis and fastened to the press plate, whereby a leading end portion of a stencil sheet is clamped between the base portion and the press plate together with the elastic film belt extended along the upper surface of the base portion when the press plate is in the closed position, while the elastic film belt floats up above the upper surface of the base portion due to a contraction of the distance between the opposite ends thereof when the press plate is in the open position, so that thereby the leading end portion of the stencil sheet is lifted up from the base portion at a time of discharging the stencil sheet, so as to facilitate and ensure the removal and discharge of the stencil sheet from the stencil clamping means of the printing drum.
Further, in Japanese Patent Laid-open Publication 5-69648 based upon an application filed by the same assignee as the present application there has been proposed a stencil sheet clamping construction of a stencil printing drum improved from the construction by the first-mentioned prior proposal, wherein the elastic film belt is constructed to have a larger modulus of elasticity in its first portion including the above-mentioned first end portion than in its second portion including the above-mentioned second end portion, or the elastic film belt is constructed to include a cantilever type auxiliary piece extending from an intermediate portion between the first and second end portions toward the side of the second end portion, so as to modify the floating up shape of the elastic film belt to be more effective for lifting up the leading end portion of the stencil sheet.
In order that the function of the elastic film belt forming as essential part of the present invention is understood, the stencil sheet clamping construction of the stencil printing drum according to the above-mentioned Japanese Patent Laid-open Publication 61-104854 is described with reference to the accompanying FIG. 1 which is an overall perspective view of the stencil printer and FIGS. 2-4 which are side views of the stencil clamping construction in an enlarged scale, showing three different operating conditions.
In these figures, 1 is a stencil printing drum of a rotary stencil printer, having a construction that annular guide portions 2 provided at opposite axial ends thereof are guided along an annular passage by two sets of support rollers 3 which are supported by a frame of the printer not shown in the figure, so that the stencil printing drum rotates about its central axis. The drum 1 is provided with a base portion 4 at a portion of its circumferential surface along a generatrix thereof, an upper surface 5 of the base portion providing one of the surfaces for clamping a leading end portion of the stencil sheet mounted to the drum. A magnet plate 6 is planted in the upper surface, so that a principal portion of the upper surface 5 is provided by an upper surface 6a of the magnet plate 6. A press plate 8 is pivotably mounted to the base portion 4 by bearing portions 7 provided at opposite ends of the base portion 4 via a shaft 9 supported by the bearing portions and supporting the press plate 8 along an inner edge thereof, so that the press plate 8 is pivotable about a pivot axis A extending through the center of the shaft 9. A pinion 10 is mounted to one end of the shaft 9, the pinion being adapted to engage with a rack not shown in the figure, so that thereby the press plate 8 is selectively driven between a closed position such as shown in FIG. 3, where the press plate is laid over the upper surface 5 of the base portion 4, and an open position such as shown in FIG. 4, where the press plate 8 is held by a magnet plate 11 planted in an opposite side portion of the base portion 4.
Above the base portion 4, there is provided an elastic film belt 13 extended circumferentially relative to the stencil printing drum over the upper surface 5 of the base portion, with a first end portion thereof being fastened to the base portion at its edge portion 12 remote from the pivot axis of the press plate and a second end portion thereof opposite to the first end portion being turned around the pivot axis A through the lower side of the shaft 9 and fastened to the press plate. By such a construction that the elastic film belt 13 extending circumferentially relative to the stencil printing drum is fastened to the edge portion 12 of the base portion 4 at the first end portion thereof and to the press plate 5 at the second end portion thereof after having turned around the pivot axis A of the press plate 8, when it is so constructed that the elastic film belt 13 is just completely extended when the press plate 8 is in the closed position laid over the upper surface 5 of the base portion as shown in FIG. 3, and when the press plate 8 is turned oppositely to the open position as shown in FIG. 1, 2 or 4, the second end portion of the elastic film belt 13 is pushed out toward the first end portion, with the elastic film belt 13 floating up arcuately above the upper surface 5 of the base portion 4 between the opposite ends thereof as shown in those figures, due to a contraction of the distance between the opposite end portions thereof.
In the stencil sheet clamping construction of a stencil printing drum according to the above-mentioned prior proposal, when a new stencil sheet is mounted to the printing drum, the press plate 8 is set at the open position as shown in FIG. 2, while a stencil sheet S is transferred by a pair of rollers 14 and 15 and guided by a guide roller 16 so that its leading end portion Sa is just positioned above the base portion 4, although, since the elastic film belt 13 is arcuately lifted up above the upper surface 5 of the base portion, the leading end portion Sa of the stencil sheet is in a condition floated above the upper surface 5 of the base portion and supported by the arcuately lifted elastic film belt 13. Starting from such a condition, when the press plate 8 is driven toward the closed position by the shaft 9, turning about the pivot axis A, the elastic film belt 13 is extended along the upper surface 5 of the base portion 4 as shown in FIG. 3, whereupon the leading end portion Sa of the stencil sheet is clamped between the base portion 4 and the press plate 8 together with the elastic film belt 13 which has just been flatly extended.
When the printing has ended and the used stencil sheet is to be discharged from the printing drum, the press plate 8 is driven again to the open position as shown in FIG. 4. According to the turn of the press plate 8 toward the open position, the elastic film belt 13 floats up above the upper surface 5 of the base plate 4 in the same manner as shown in FIG. 2, so that thereby the leading end portion Sa of the stencil sheet is lifted up from the upper surface 5 of the base portion 4. When the printing drum is rotated counter-clockwise in the figure, the leading end portion Sa of the stencil sheet is scooped up by a discharge claw not shown in the figure, so that the stencil sheet is transferred toward a discharge means, ensuring a definite operation of the stencil discharge means.
The elastic film belt 13 certainly operates to lift up the leading end portion Sa of the stencil sheet above the upper surface 5 of the base portion at discharging of the stencil sheet, as shown in FIG. 4, so that when the printing drum is rotated from this state in the counter-clock wise direction, the leading end portion Sa of the stencil sheet is definitely scooped up by a discharge claw not shown in the figure to be transferred along a stencil discharge passage. However, when the stencil sheet is mounted, as shown in FIG. 2, the elastic film belt 13 arcuately floating up above the upper surface 5 of the base portion 4 does no effective operation, but rather presents a surface traversing a forward movement of the leading end of the stencil sheet transferred by the pair of rollers 14 and 15, shown by a broken line in FIG. 2, thereby adversely affecting a smooth placing of the leading end portion Sa of the stencil sheet upon the upper surface 5 of the base portion 4.